Probe pins are mounted in a socket and a probe card as contacts to apply electrical signals or voltages to test objects. Generally, the probe pins include a hollow cylindrical barrel, a pair of plungers that is attached to both ends of the barrel, and a coil spring received in the barrel for supporting the plungers elastically. There are two types for such probe pins. One is a single-ended probe. One of the plungers slides in the axial direction of the barrel, and the other is a double-ended probe. Both plungers slide in the axial direction of the barrel.
Semiconductors are tested to test whether each device passes or fails before shipping as final products. For example, circuit elements in the semiconductor wafer are tested by the prove card using probe pins. A package, which includes plastic molding surrounding a semiconductor chip that was cut away from the wafer, is tested by the socket installing the probe pins.
For example, when a BGA (Ball Grid Array) type package is mounted in the socket, one plunger is contacted with the solder ball, and the other plunger is connected to an electrode or conductive land on the printed wiring board mounting the socket. Electrical signals are supplied with the electrodes on the printed wiring board from the test equipment and are provided to the solder balls from the other plunger to one plunger through the barrel. Electrical currents flowing on the probe pins are mostly paths from the plungers through the barrel, and these current paths determine substantial resistance of the probe pin.
High integration and high density of the semiconductor chip cause an increase in the number of solder balls (external terminals). The miniaturization of a package causes a narrowing of the pitch of the solder balls. Accordingly, the pitch of the probe pins mounted in the socket should be matched with that of the solder balls, so that the miniaturization of the probe pins is required. To miniaturize the probe pins, the diameter of the barrel should be reduced, and the diameter of the plunger should be reduced so as to be adjusted.
Increasing the number of probe pins mounted in the socket requires reducing electrical resistance of the probe pins naturally and minimizing the frustrations of the electrical resistances between the probe pins. Reducing the diameter of the barrel causes the barrel to increase the electrical resistance, and in addition, it becomes difficult to maintain stable contacts between the barrel and the plungers. The insufficient contacts between the barrel and the plungers result in increasing the resistance of the probe pins, and the unstable contacts cause a change in the resistance of the probe pins. Accordingly, stable and constant contact between the barrel and the plungers are needed.
To stabilize the contacts between the plungers and the barrel, it is effective to strengthen the contacting pressure of the plungers in the perpendicular direction with respect to the barrel surface. To perform this, the methods of improving the plunger or spring can be considered.
One typical method of improving the plunger is called bias-cut; the contacting surface of the plunger with the spring is cut so as to be sloped, thus the spring presses the plunger in the oblique direction with respect to the axis of the probe pin and the vertical pressure with respect to the barrel is strengthened.
FIG. 1A shows a sectional view of the probe pin with a bias cut type. As illustrated in FIG. 1A, the probe pin includes a pair of plungers 10, 20, a spring 30 and a barrel 40. The surfaces 12, 22 of the plungers 10, 20 contacting with spring 30 are sloped. The axial force by the spring 30 presses the plungers 10, 20 vertically with respect to the axis direction and applies a constant pressure of contacts between the plungers 10, 20 and the barrel 40.
A method of improving a spring is described in WO 2006/007440 A1, which discloses a method to increase the vertical pressure of the plunger against the barrel by offsetting a central axis of the spring. As shown in FIG. 1B, the probe pin includes the plungers 10, 20, a spring 50 and the barrel 40. The contacting surfaces 14, 24 of the plungers 10, 20 are ordinary conical shape, namely, wedge type, while the spring 50 includes a first portion 52 having a first central axis and a second portion 54 having a second central axis, the second portion 54 is adjacent to the first portion 52 and its second central axis is offset from the first central axis. Such an offset spring 50 allows the plungers 10, 20 to be energized in the vertical direction with respect to the axis direction, thereby maintaining the stable contacts between the plungers 10, 20 and the barrel 40.